Microorganisms are ubiquitous in our environment. Although many are harmless and even beneficial, some are well-known pathogens, while others can elicit allergenic responses in human. The World Health Organization (WHO) reported that one of the most common routes for transmission of infectious diseases is by indirect contact with surfaces contaminated with infectious droplets produced by the patients' coughing, sneezing or talking. Many microbes, including viruses can survive for days on surfaces. For instance, influenza virus can remain viable for 24-48 hours, while parainfluenza and SARS viruses are known to survive for hours and days on most surfaces. Some pathogens are known to transmit through fomites. When fomites touch contaminated surfaces by pathogens, the pathogens transmit through the fomites. Therefore, regular cleaning and disinfection are important for breaking the chain of infection, and the use of antimicrobial surface coating provides additional safeguard against the disease transmission.
Many metals including silver, copper and brass possess intrinsic germicidal properties that can kill many pathogenic microorganisms upon contact [Fang, H. P., Pure & Appl. Chem. 1997, 69, 2425-2429]. Nanosilvers, photocatalytic TiO2 and surface tethered bactericides (e.g., quarternary ammonium compounds (QACs), phosphonium salts) are known to be used as contact-killing antimicrobial surface coatings in recent years. However, the anti-microbial properties of these materials diminish rapidly when the surface is fouled by dirt and contaminants, thus requiring frequent cleaning to maintain their effectiveness.
It has also been developed to store antibiotics, biocides (e.g., phenols, halogens) and metals (e.g., silver ions) in bulk materials and coatings for a slow, gradual and continuous release into the environment providing a sustained “release-killing” until the content is exhausted. Cohen's group (Li, Z. et al., Langmuir 2006, 22, 9820-9823) employed layer-by-layer, self-assembly method to produce a two-level antibacterial coating with both “release-killing” and “contact-killing” capabilities based on stored silver salt and surface grafted quaternary ammonium, while Ho et al. (Ho, C. H. et al., Adv. Mater 2004, 16, 957-961) used a polymer film to immobilize nanosilver to achieve both “contact- and release-killing” effects, while a grafted layer of polyethylene glycol repelled the adhesion of bacteria. However, the adhesion of pathogens (such as microorganisms), dirt and/or contaminants on surfaces is still a problem in the antimicrobial coating with the both contact- and release-killing effects. Accordingly, a multi-level antimicrobial surface coating is still desired.